BackboneJS : Models

Models are the heart of any JavaScript application, containing the interactive data as well as a large part of the logic surrounding it: conversions, validations, computed properties, and access control. You extend Backbone.Model with your domain-specific methods, and Model provides a basic set of functionality for managing changes.

The following is a contrived example, but it demonstrates defining a model with a custom method, setting an attribute, and firing an event keyed to changes in that specific attribute. After running this code once, sidebar will be available in your browser's console, so you can play around with it.

extendBackbone.Model.extend(properties, [classProperties]) To create a Model class of your own, you extend Backbone.Model and provide instance properties, as well as optional classProperties to be attached directly to the constructor function.

extend correctly sets up the prototype chain, so subclasses created with extend can be further extended and subclassed as far as you like.

Brief aside on super: JavaScript does not provide a simple way to call super — the function of the same name defined higher on the prototype chain. If you override a core function like set, or save, and you want to invoke the parent object's implementation, you'll have to explicitly call it, along these lines:

constructor / initializenew Model([attributes], [options]) When creating an instance of a model, you can pass in the initial values of the attributes, which will be set on the model. If you define an initialize function, it will be invoked when the model is created.

If you pass a {collection: ...} as the options, the model gains a collection property that will be used to indicate which collection the model belongs to, and is used to help compute the model's url. The model.collection property is normally created automatically when you first add a model to a collection. Note that the reverse is not true, as passing this option to the constructor will not automatically add the model to the collection. Useful, sometimes.

If {parse: true} is passed as an option, the attributes will first be converted by parse before being set on the model.

getmodel.get(attribute) Get the current value of an attribute from the model. For example: note.get("title")

setmodel.set(attributes, [options]) Set a hash of attributes (one or many) on the model. If any of the attributes change the model's state, a "change" event will be triggered on the model. Change events for specific attributes are also triggered, and you can bind to those as well, for example: change:title, and change:content. You may also pass individual keys and values.

escapemodel.escape(attribute) Similar to get, but returns the HTML-escaped version of a model's attribute. If you're interpolating data from the model into HTML, using escape to retrieve attributes will prevent XSS attacks.

hasmodel.has(attribute) Returns true if the attribute is set to a non-null or non-undefined value.

if (note.has("title")) {
...
}

unsetmodel.unset(attribute, [options]) Remove an attribute by deleting it from the internal attributes hash. Fires a "change" event unless silent is passed as an option.

clearmodel.clear([options]) Removes all attributes from the model, including the id attribute. Fires a "change" event unless silent is passed as an option.

idmodel.id A special property of models, the id is an arbitrary string (integer id or UUID). If you set the id in the attributes hash, it will be copied onto the model as a direct property. Models can be retrieved by id from collections, and the id is used to generate model URLs by default.

idAttributemodel.idAttribute A model's unique identifier is stored under the id attribute. If you're directly communicating with a backend (CouchDB, MongoDB) that uses a different unique key, you may set a Model's idAttribute to transparently map from that key to id.

cidmodel.cid A special property of models, the cid or client id is a unique identifier automatically assigned to all models when they're first created. Client ids are handy when the model has not yet been saved to the server, and does not yet have its eventual true id, but already needs to be visible in the UI.

attributesmodel.attributes The attributes property is the internal hash containing the model's state — usually (but not necessarily) a form of the JSON object representing the model data on the server. It's often a straightforward serialization of a row from the database, but it could also be client-side computed state.

Please use set to update the attributes instead of modifying them directly. If you'd like to retrieve and munge a copy of the model's attributes, use _.clone(model.attributes) instead.

Due to the fact that Events accepts space separated lists of events, attribute names should not include spaces.

changedmodel.changed The changed property is the internal hash containing all the attributes that have changed since its last set. Please do not update changed directly since its state is internally maintained by set. A copy of changed can be acquired from changedAttributes.

defaultsmodel.defaults or model.defaults() The defaults hash (or function) can be used to specify the default attributes for your model. When creating an instance of the model, any unspecified attributes will be set to their default value.

Remember that in JavaScript, objects are passed by reference, so if you include an object as a default value, it will be shared among all instances. Instead, define defaults as a function.

toJSONmodel.toJSON([options]) Return a shallow copy of the model's attributes for JSON stringification. This can be used for persistence, serialization, or for augmentation before being sent to the server. The name of this method is a bit confusing, as it doesn't actually return a JSON string — but I'm afraid that it's the way that the JavaScript API for JSON.stringify works.

syncmodel.sync(method, model, [options]) Uses Backbone.sync to persist the state of a model to the server. Can be overridden for custom behavior.

fetchmodel.fetch([options]) Merges the model's state with attributes fetched from the server by delegating to Backbone.sync. Returns a jqXHR. Useful if the model has never been populated with data, or if you'd like to ensure that you have the latest server state. Triggers a "change" event if the server's state differs from the current attributes. fetch accepts success and error callbacks in the options hash, which are both passed (model, response, options) as arguments.

savemodel.save([attributes], [options]) Save a model to your database (or alternative persistence layer), by delegating to Backbone.sync. Returns a jqXHR if validation is successful and false otherwise. The attributes hash (as in set) should contain the attributes you'd like to change — keys that aren't mentioned won't be altered — but, a complete representation of the resource will be sent to the server. As with set, you may pass individual keys and values instead of a hash. If the model has a validate method, and validation fails, the model will not be saved. If the model isNew, the save will be a "create" (HTTP POST), if the model already exists on the server, the save will be an "update" (HTTP PUT).

If instead, you'd only like the changed attributes to be sent to the server, call model.save(attrs, {patch: true}). You'll get an HTTP PATCH request to the server with just the passed-in attributes.

Calling save with new attributes will cause a "change" event immediately, a "request" event as the Ajax request begins to go to the server, and a "sync" event after the server has acknowledged the successful change. Pass {wait: true} if you'd like to wait for the server before setting the new attributes on the model.

In the following example, notice how our overridden version of Backbone.sync receives a "create" request the first time the model is saved and an "update" request the second time.

save accepts success and error callbacks in the options hash, which will be passed the arguments (model, response, options). If a server-side validation fails, return a non-200 HTTP response code, along with an error response in text or JSON.

book.save("author", "F.D.R.", {error: function(){ ... }});

destroymodel.destroy([options]) Destroys the model on the server by delegating an HTTP DELETE request to Backbone.sync. Returns a jqXHR object, or false if the model isNew. Accepts success and error callbacks in the options hash, which will be passed (model, response, options). Triggers a "destroy" event on the model, which will bubble up through any collections that contain it, a "request" event as it begins the Ajax request to the server, and a "sync" event, after the server has successfully acknowledged the model's deletion. Pass {wait: true} if you'd like to wait for the server to respond before removing the model from the collection.

book.destroy({success: function(model, response) {
...
}});

Underscore Methods (9) Backbone proxies to Underscore.js to provide 9 object functions on Backbone.Model. They aren't all documented here, but you can take a look at the Underscore documentation for the full details…

validatemodel.validate(attributes, options) This method is left undefined and you're encouraged to override it with any custom validation logic you have that can be performed in JavaScript. By default save checks validate before setting any attributes but you may also tell set to validate the new attributes by passing {validate: true} as an option. The validate method receives the model attributes as well as any options passed to set or save. If the attributes are valid, don't return anything from validate; if they are invalid return an error of your choosing. It can be as simple as a string error message to be displayed, or a complete error object that describes the error programmatically. If validate returns an error, save will not continue, and the model attributes will not be modified on the server. Failed validations trigger an "invalid" event, and set the validationError property on the model with the value returned by this method.

urlmodel.url() Returns the relative URL where the model's resource would be located on the server. If your models are located somewhere else, override this method with the correct logic. Generates URLs of the form: "[collection.url]/[id]" by default, but you may override by specifying an explicit urlRoot if the model's collection shouldn't be taken into account.

Delegates to Collection#url to generate the URL, so make sure that you have it defined, or a urlRoot property, if all models of this class share a common root URL. A model with an id of 101, stored in a Backbone.Collection with a url of "/documents/7/notes", would have this URL: "/documents/7/notes/101"

urlRootmodel.urlRoot or model.urlRoot() Specify a urlRoot if you're using a model outside of a collection, to enable the default url function to generate URLs based on the model id. "[urlRoot]/id" Normally, you won't need to define this. Note that urlRoot may also be a function.

parsemodel.parse(response, options)parse is called whenever a model's data is returned by the server, in fetch, and save. The function is passed the raw response object, and should return the attributes hash to be set on the model. The default implementation is a no-op, simply passing through the JSON response. Override this if you need to work with a preexisting API, or better namespace your responses.

If you're working with a Rails backend that has a version prior to 3.1, you'll notice that its default to_json implementation includes a model's attributes under a namespace. To disable this behavior for seamless Backbone integration, set:

ActiveRecord::Base.include_root_in_json = false

clonemodel.clone() Returns a new instance of the model with identical attributes.

isNewmodel.isNew() Has this model been saved to the server yet? If the model does not yet have an id, it is considered to be new.

hasChangedmodel.hasChanged([attribute]) Has the model changed since its last set? If an attribute is passed, returns true if that specific attribute has changed.

Note that this method, and the following change-related ones, are only useful during the course of a "change" event.

changedAttributesmodel.changedAttributes([attributes]) Retrieve a hash of only the model's attributes that have changed since the last set, or false if there are none. Optionally, an external attributes hash can be passed in, returning the attributes in that hash which differ from the model. This can be used to figure out which portions of a view should be updated, or what calls need to be made to sync the changes to the server.

previousmodel.previous(attribute) During a "change" event, this method can be used to get the previous value of a changed attribute.

previousAttributesmodel.previousAttributes() Return a copy of the model's previous attributes. Useful for getting a diff between versions of a model, or getting back to a valid state after an error occurs.